Hospital units that don’t have enough nurses can be hazardous to your health.

New research from the Mayo Clinic in Rochester, Minn., suggests a patient’s risk of death increases by 2 percent per hospital shift when units are understaffed with registered nurses.

The study, published today in the New England Journal of Medicine, examines staffing at just one hospital over four years. It finds that nursing levels met or were close to targets in about 84 percent of all shifts. And while researchers said the rate was good news for most patients at the hospital — because overall mortality was lower than expected — it spelled trouble for patients in units considered understaffed.

In a few cases, patients saw their risk of death increase by about 25 percent because they happened to experience between 10 and 14 under-staffed shifts during just five days in the hospital.

The findings promise to stir the debate over nurse staffing levels in U.S. medical centers. The issue was at the heart of a one-day nurses strike against about a dozen hospitals last year in the Twin Cities.

“Hospitals need to know what staffing is needed to care for their patients and then take actions to make sure those staffing levels are achieved,” said Marcelline Harris, a Mayo Clinic researcher. “Your risk of mortality increases with increasing number of shifts that are below your (staffing) target.”

The report looks at the nurse staffing issue in the context of a single medical center — the Mayo Clinic’s hospitals in Rochester. The hospitals admitted nearly 200,000 patients from 2003 to 2006. Similar studies have looked at many different hospitals, a method that critics say fails to control for issues such as the quality of doctors and access to technology.

While the overall death rate for all patients in the Mayo study was lower than expected, it varied along with patients’ exposure to under-staffed units. Researchers said a shift was understaffed when the number of hours work ed by registered nurses on the unit was at least eight hours below the target staffing level.

Of all patients evaluated during the first 30 days after admission, about one-third stayed in units in which no shifts had actual staffing levels that were below target. But about 35 percent of patients experienced at least three shifts where staffing targets set by hospital management weren’t met.

“Largely, managers are taking a lot of responsibility to meet those targets,” Harris said. “When patients started to have three or more shifts with below-target staffing, that is when we started to really see this risk increase.”

Some degree of under-staffing is difficult to avoid in hospitals, she said. That’s because nurses sometimes call in sick and the number of patients needing care can change unexpectedly.

The study also found that mortality risks increased as patients were cared for in units experiencing “high turnover,” where demands on nursing staff increa! sed with a high number of admissions, transfers and discharges. Risk of death increased 4 percent for each high-turnover shift for which a patient was exposed.

About 40 percent of patients were not exposed to high-turnover shifts while about 13 percent were exposed to three or more shifts with high turnover.

“For hospitals that generally succeed in maintaining RN staffing levels that are consistent with each patient’s requirements for nursing care, this study underscores the importance of flexible staffing practices,” the study authors wrote. “Nurse staffing models that facilitate shift-to-shift decisions … are an important component of the delivery of care.”

Hospitals said they welcomed the study, as the need for flexibility was a central argument made by hospitals in last year’s contract negotiations with unionized hospital nurses in the Twin Cities.

The finding that patient turnover can affect mortality rates illustrates why it might be difficult to write nurse staffing rules into contracts and state laws,! said Lo well Taylor, a professor of economics at Carnegie Mellon University in Pittsburgh.

“What this shows is that other factors matter — such as the turnover of patients — that are subtle and would be difficult to mandate in clearly specified work rules,” said Taylor, who has studied a California law that mandates nurse-to-patient ratios.

But Carol Diemert of the Minnesota Nurses Association said staffing rules should be written in ways that allow for patient turnover. The issue just hasn’t been given the attention it deserves, she said.

More broadly, nurse union officials said the study supports many of the points organized nurses made in last year’s contract fight. The fact that about 16 percent of all shifts documented in the study fell short of staffing targets suggests a lack of accountability on the part of hospitals, said nurse Deb Haugen, a union official.

“What we’re seeing from this study is that on a day-to-day, shift-to-shift basis at an elite! hospital, they’re not meeting their own standards,” Diemert said. “They’re one whole nurse short per shift. … When you’re down one nurse, it has an impact.”